Gain-of-function Shh mutants activate Smo in cis independent of Ptch1/2 function

Abstract

Sonic Hedgehog (Shh) signaling is characterized by strict non-cell autonomy; cells expressing Shh do not respond to their ligand. Here, we identify several Shh mutations that gain the ability to activate the Hedgehog (Hh) pathway in cis. This activation requires the extracellular cysteine rich domain of Smoothened, but is otherwise independent of Ptch1/2. Many of the identified mutations disrupt either a highly conserved catalytic motif found in peptidases or an a-helix domain frequently mutated in holoprosencephaly-causing SHH alleles. The expression of gain-of-function mutants often results in the accumulation of unprocessed Shh pro-peptide, a form of Shh we demonstrate is sufficient to activate the Hh response cell-autonomously. Our results demonstrate that Shh is capable of activating the Hh pathway via Smo independently of Ptch1/2, and that it harbors an intrinsic mechanism that prevents cell-autonomous activation of the pathway to favor non-cell autonomous signaling.